Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Stack Overflow - Community-based Q&A part of the Stack Exchange platform.
KanbanFlow - KanbanFlow is a Lean project management tool allowing real-time collaboration between team members. Supports the Pomodoro technique for time tracking.
The Answer Bank - The Answer Bank is the UK’s leading question-and-answer site.
Meistertask - MeisterTask is an intuitive task management and collaboration tool.
Blurtit - Blurtit is a Q&A website. You can ask a question or answer on any question.